FR2895924A1 - METHOD FOR BRAKING BETWEEN AT LEAST TWO STACKED BODIES - Google Patents

Abstract

In accordance with this soldering method, a laser (24) is directed on an end face (F) of the cell (18) so that the laser (24) heats the cell (18). At least one parameter of the laser (24) is set to a value which is the image by a mathematical model of at least one thermal characteristic of the stack (18). The laser parameter (24) is a parameter selected from an irradiation time, an irradiation surface of the end face of the cell by the laser and an irradiation power of the aser (24).

Description

The present invention relates to a brazing process between them from

  at least two stacked elements, a method for producing a mathematical model for implementing the soldering method and an electronic module. It applies more particularly to the brazing of an electronic module comprising a first support element, for example a printed circuit, on which are stacked at least second and third elements forming electrical organs, the three elements being brazed together. Stacking the electronic components on the support makes it possible to obtain a relatively compact electronic module.

  Already known in the state of the art a brazing process between them of at least two stacked elements forming a stack, of the type in which a laser is directed on an end face of the stack so that the The laser heats the battery and adjusts at least one parameter of the laser. Usually, the laser parameter is a parameter selected from an irradiation time, an irradiation area of the end face of the cell by the laser and a laser irradiation power. The stack comprises, for example, a printed circuit support (first element of the battery) and a semiconductor chip (second element of the battery). Between these two elements is interposed a solder mass.

  The stack is characterized by certain thermal features, rotatically the critical temperature of the semiconductor chip beyond which the pellet bed is destroyed and the solder melting temperature which must be lower than the critical temperature of the semiconductor chip . In general, each parameter of the laser is adjusted empirically to a value such that, on the one hand, the maximum temperature reached of the cell does not exceed the critical temperature of the semiconductor chip and, on the other hand, that the maximum temperature reached of the cell is greater than the melting temperature of the solder mass. Thus, in the case of a battery comprising only two elements, the setting of the value of each parameter of the laser can be achieved empirically because the number of thermal characteristics of the battery to be taken into account is limited. However, the number of thermal characteristics that must be taken into account increases with the number of elements in the stack. Therefore, when the stack comprises at least three elements, an empirical setting of the parameter value is too random to take into account all the thermal characteristics of the cell and can lead to a quality brazur3.

  -2- relatively poor between two elements of the stack or damage to one of the elements of the stack. The object of the invention is to propose a brazing process for at least two stacked elements in which the adjustment of the value of the laser parameter is relatively fast and accurate, regardless of the number of elements in the stack. . For this purpose, the object of the invention is a brazing process of the aforementioned type, characterized in that the parameter is set to a value which is the image by a mathematical model of at least one thermal characteristic of the stack. . Thus, advantageously, it is possible to quickly and accurately determine the value of the laser parameter using the mathematical model. The mathematical model can be implemented in the form of a computer program providing an accurate and adapted laser parameter setting value, in contrast to conventional empirical methods. The soldering method according to the invention may further comprise the characteristics: the parameter of the laser is a parameter chosen from an irradiation time, an irradiation surface of the end face of the cell by the laser and a irradiation power of the laser; the thermal characteristic of the cell is chosen from a critical temperature of damage to an element of the cell, a melting point of a solder mass interposed between two elements of the cell and a temperature of a synthetic material bordering one of the elements of the pile. The subject of the invention is also a process for producing a mathematical model for carrying out the brazing process according to the invention, characterized in that it comprises the following steps, which conform to an experimental plan: adjusting the value of the laser parameter, - measuring temperatures over an area of at least one sample of at least a portion of the stack during a period of irradiation of a first end face of the sample by the laser, selecting at least a remarkable temperature from the temperatures measured, and in that it also comprises a step of defining the mathematical model so that a vector having at least the coordinate value

  -3- of the laser parameter is the image by the mathematical model of a vector having at least coordinate the remarkable temperature. A method for producing a mathematical model according to the invention may further comprise the following characteristics: the mathematical model comprises at least one mathematical function of polynomial type; the zone extends on a second end face of the sample and the remarkable temperatures are maximum temperatures observed in a remarkable sub-zone of this zone; the remarkable sub-zone separates a part of the zone intended to be covered with a solder mass from another part of the zone intended not to be covered with the solder mass; the sub-zone is the projection on the second end face of the sample of the irradiation surface of the laser; an element of the sample comprising a metal part bordered by a part made of synthetic material, the remarkable sub-area separating the metal part from the part made of synthetic material; - prior to the temperature reading step, blacken the sample area; the temperatures are recorded from a treatment of an infrared image of the sample area; the experimental plane is a plane of the type chosen from a Behnken E3ox plane, a composite central plane and a D-optimal plane; the mathematical model is determined from a method for analyzing the variance of the remarkable temperatures. The subject of the invention is also an electronic module comprising a first support element on which at least second and third elements forming electrical elements are stacked, the three elements being brazed together, characterized in that the three elements are brazed together. by irradiating the electronic module with a laser. The invention will be better understood on reading the description which follows, given solely by way of example and with reference to the drawings in which: FIG. 1 is a schematic view of an electronic module according to FIG. invention; FIGS. 2 to 4 are schematic views of samples of the electronic module of FIG. 1 at different stages of a method;

  -4- of a mathematical model for the implementation of a brazing process according to the invention. FIG. 1 shows an electronic module according to the invention. This electronic module is designated by the general reference 10.

  The electronic module 10 comprises a first element 12. In the example described, the first element 12 forms a support. In the example illustrated, the support 12 comprises a metal PVI portion partially bordered by a PS portion of synthetic material (plastic). The support 12 may advantageously be a printed circuit substrate.

  Alternatively, the support 12 may be made of a ceramic material. The electronic module 10 also comprises second 14 and third 16 elements stacked along a vertical axis X on the metal part PM of the support 12 so that the three elements 12, 14 and 16 form a stack 18. The second and third elements 14 and 16 form, by way of example, semi-conductive pellet electronic devices. The elements of the cell 18 are brazed together according to a soldering method according to the invention by irradiation of the electronic module 10 by a laser 24. Thus, in the illustrated example, first 20 and second 22 solder masses are interposed respectively between the support 12 and the pellet 14 and between the pellet 14 and the pellet 16. In a conventional manner, the solder materials 20 and 22 are dough masses or masses cut in a ribbon and reported between the elements 12 to 16 of the battery 18 before soldering. In a variant, the solder-forming masses may be formed by coatings of the pellets 14 and 16, integral with these pellets before soldering, the coating material being in this case silver, gold, silver, Pewter, etc. The heat, for melting the solder masses 20 and 22 and thus brazing the three elements together, is created by the irradiation of the electronic module 10 by a laser 24 emitted by a laser source (not shown).

  The laser source may comprise, for example, a laser diode emitting a laser beam, in particular of the infrared type. The laser 24 is directed on an end face F1 of the cell 18 so as to heat the cell 18. More particularly, the laser 24 is focused by means 26 for focusing the laser 24 on an irradiation surface S of the laser. PM metal part of the support 12 by the laser 24.

  In order to sufficiently heat the cell 18 to melt the brazing masses 20 and 22 without damaging any of the three elements of the cell 18, the laser 24 is provided with adjustment means (not shown) with a value of at least one parameter of the laser 24.

  In the example described, the parameters of the laser 24 are a duration: of irradiation D, the irradiation surface S by the laser 24 of the end face F1 and a irradiation power P of the laser 24. The values parameters D, S, P of the laser 24 are adjusted so as to take into account the thermal characteristics of the battery 18, in particular the melting temperatures of the brazing masses 20 and 22, the critical temperatures of the three elements of the battery 18 (when an element is heated beyond its critical temperature, it is destroyed), as well as the melting temperature of the synthetic material bordering the support 12. For example, the melting point of the brazing masses is of the order of 300 C and the critical temperatures of the three elements of the battery 18 are of the order of 500 C. In accordance with the soldering method of the invention, each parameter of the laser 24 is adjusted to a value which is the image by a mathematical model M of at least one thermal characteristic of the battery 18. A method of producing the mathematical model M according to the invention will be described below, with reference to FIGS. 2 to 4. In order to carry out the mathematical model M, the following steps, in accordance with a experimental plan. In a manner known per se, an experimental design makes it possible to define a number N of experiments to be performed in order to obtain a sufficiently accurate mathematical model, as well as to define for each experiment the values of the parameters to be adjusted. For example, the experimental plan is a plane of the type chosen from a Behnken Box plan, a composite central plan and a D-optimal plan. Other conventional experimental designs can be used. Thus, in the example described, the experimental plane defines a set E of parameter values of the laser 24 comprising: a number ND of irradiation time D values D of the laser between a minimum value Dmin and a maximum value Dmax, for example between 200 ms and 700 ms, a number Np of values Pi of irradiation power P between a minimum value Pmin and a maximum value Pmax, for example between 700W and 2000W,

  A number NS of values S 1 of irradiation area S between a minimum value Smin and a maximum value Smax. Thus, the experimental design defines the number N of experiments to be performed, these N experiments being chosen, for example, from the possible combinations of the different values of the parameters of the laser, for example N = ND x Np × N; in the case of a so-called complete experimental design or N <ND x Np x NS in the case of a fractional experimental design, in particular of the Box Behnken, central composite or D-optimal type. As a result, for each experiment of the experimental plane, the parameters D, S, P of the laser are respectively adjusted to values Di, S, P; in the set E defined by the experimental plan. In a first step, a first sample 28 is formed of a portion of the stack 18 (FIG. 2). It can be seen in FIG. 2 that the first sample 28 comprises only the first element of the battery 18, namely the support 12.

  The first sample 28 comprises a first end face FI corresponding to the lower face of the support 12 and a second end face F2 corresponding to the upper face of the support 12. The N experiments defined by the experimental plan are then carried out on this surface. first sample 28.

  For each experiment, the parameters D, S, P of the laser 24 are respectively adjusted to values D 1, S 1, Pi of the set E. Then a parameter vector VP is defined; having for first, second and third co-ordinates, respectively, the values D1, Si and P1 of the three parameters D, S, P of the laser 24. The laser 24 is directed on the first end face F1 during an irradiation duration D ( set to the value D;) of this face F1 by the laser 24 so that the laser 24 heats the first sample 28. In a first step, temperatures are then recorded on a zone Z1 of the first sample 28 In the illustrated example, the zone Z1 extends on the second end face F2 opposite to the first end face F1 of the first sample 28. Preferably, the temperatures are read from a treatment of an infrared image of the zone Z1 of the sample 28. The infrared image is obtained by conventional means of acquiring an infrared image comprising, for example, an infrared camera 30.

  The camera 30 is, for example, provided with a lens 32 with a large field angle adapted to the size of the zone Z1 and with a magnification sufficient to obtain a good resolution of the relevant elements of the zone Z1. Moreover, the camera 30 allows, for example, an acquisition of the images at a frequency of the order of 50 Hz. Thus, for an irradiation time C) of the first sample 28 by the laser 24 of a value D, = 500 ms, it is possible to obtain 25 images of zone Z1. Preferably, before raising the temperatures on the zone Z1 of the first sample 28, the zone Z1 is blackened (for example by covering it with a black coating (paint, etc.)) so as to form a black body. In a second step, remarkable temperatures are selected from the temperatures measured. Preferably, the remarkable temperatures are three maximum temperatures found respectively in three remarkable subzones SZ1A, SZ1B, SZ1 C of the zone Z1 of the first sample 28. In fact, the distribution of temperatures is not uniform in the first sample 28 and the temperatures are higher near the irradiation surface of the cell 18 by the laser 24. The first remarkable subzone SZ1A separates a portion P1 from the zone Z1 intended to be covered with the solder mass. 20 of another part F'2 of the zone Z1 intended not to be covered with the solder mass 20. The second sub-area SZ1B remarkable is the projection on the second end face F2 of the first sample 28 of the S irradiation surface of the laser. The third subzone SZ1 C remarkable separates the metal part PM of the synthetic material part PS. For each experiment, three first coordinates of a temperature vector VT are defined as being the three maximum temperatures respectively of the subzones SZ1A, SZ1B, SZ1 C corresponding to the setting of the laser parameters Ju to the values D ;, Si and P; the VP parameter vector ;.

  In a second step, forming a second sample 34 of a portion of the stack 18 (Figure 3). It can be seen in FIG. 3 that the second sample 34 comprises only the first two elements of the battery 18, namely the support 12 and the wafer 14. In the second sample 34, the second end face F2 'corresponds to the In an analogous manner to the first sample 28, the N experiments defined by the experimental plane are carried out on this second sample 34. Thus, for each experiment, the parameters D, S, F, from the laser 24 to the values D ;, Si, P; of the set E and define a vector "parameter" VP; having for first, second and third coordinates respectively the values D ;, Si, P; three parameters D, S, P of the laser. In a first step, temperatures are recorded on an area Z2 of the second sample 34 during a irradiation period D of the first face F1 of the second sample 34 by the laser 24. The zone Z2 extends over the second end face F2 'of the end of the second sample 34. Preferably, before raising the temperatures on the zone Z2 of the second sample 34, the zone Z2 is blackened so as to form a black body. Then, in a second step, outstanding temperatures are selected from the temperatures found in zone Z2. Preferably, the remarkable temperatures are two maximum temperatures respectively recorded in two remarkable subzones SZ2A, SZ2B of the zone Z2. The first remarkable subzone SZ2A separates a part P1 from the zone Z2 intended to be covered with the solder mass 22 of another part F2 of the zone Z2, so as not to be covered with the solder mass 22. second remarkable SZ2B sub-area is the projection on the second end face F2 'of the second sample 34 of the irradiation surface S of the laser. For each experiment, fourth and fifth coordinates of the temperature vector VT are defined as the maximum temperatures respectively of the sub-areas SZ2A and SZ2B corresponding to the adjustment of the laser parameters to the values D ;, S; and P; the VP parameter vector ;. In a third step, optionally, a third sample 36 is formed of a portion of the stack 18 (FIG. 4). FIG. 4 shows that the third sample 36 comprises the three elements of the battery 18, namely the support 12 and the two semiconductor chips 14 and 16. In the third sample 36, the second face F2 "d end corresponds to the upper face of the pellet 16. In a similar manner to the first 28 and second 34 samples, carries out the 35 N experiments defined by the experimental plan on this third sample 36.

  Thus, for each experiment, the parameters D, S, P of the laser 24 are adjusted to the values D 1, Si, P; of the set E and defining a parameter vector VP, having for first, second and third coordinates respectively the values D ;, S; and P; three parameters D, S, P of the laser.

  During a first step, temperatures are recorded on a zone Z3 of the third sample 36 during a period of irradiation D of the first face F1 of the third sample 36 by the laser 24. Preferably, before raising the temperatures on the zone Z3 of the third sample 36, the zone Z3 is blackened so as to form a black body.

  In a second step, a remarkable temperature is selected from the temperatures found in zone Z3. Preferably, the remarkable temperature is a maximum temperature found in a sixth outstanding subfield SZ3 which is the projection on the second end face F2 "of the third sample 36 of the irradiation surface S of the laser.

  For each experiment, a sixth coordinate of the temperature vector VT is defined; as being the sixth coordinate the maximum temperature selected in the sixth sub-area SZ6, corresponding to a setting of the laser parameters to the values D ;, Si and P; the VP parameter vector ;. Once the N experiments have been performed on each of the three samples 28, 34 and 36, N parameter vectors VP are obtained; and N temperature vectors VT; correspondents. The mathematical model M is then defined so that each parameter vector VP; laser 24 is the image by the model M of each temperature vector VT; corresponding.

  In this example, the mathematical model M is defined from a method for analyzing the variance of the remarkable temperatures and preferably includes at least one polynomial-type mathematical function. Thus, for brazing together the three elements 12, 14, 16 of the battery 18, the procedure is as follows.

  A temperature vector VT is defined; having in the example described the first to sixth coordinates: - a temperature greater than or equal to the melting temperature of the solder mass 20, - a temperature strictly below the critical temperature of the support 12, - a temperature strictly less than the melting temperature of the synthetic material bordering the metal part PM of the support 12,

  A temperature greater than or equal to the solder mass melting temperature 22, a temperature strictly below the critical temperature of the pellet 14, and a temperature strictly below the critical temperature of the pellet 16. coordinates of this vector VT; in a computer in which the mathematical model M is implemented so as to obtain at least one image by this mathematical model M of this vector temperature VT; (the vector VT may possibly have several images by the mathematical model M). This image is a VP parameter vector; having as coordinates the values D ,, Si, P; adapted to the soldering of the battery 18. Finally, the parameters D, S, P of the laser 24 are adjusted to the values D.sub.Si, P.sub.1, which makes it possible to obtain, in a single soldering operation, the electronic module 10 illustrated on FIG. Figure 1.

  The electronic module 10 according to the invention, obtained by laser brazing, differs from an electronic module obtained by a conventional soldering method (refusal furnace, etc.) by the appearance of the solder masses. Indeed, the laser brazing causes an intense and relatively short heating followed by a relatively fast cooling. Thus, the material dan; where the solder masses are formed, solidifies to form fine dendrites.

Claims (14)

  A method of brazing together at least two stacked elements (12, 14, 16) forming a stack (18), in which a laser (24) is directed on an end face of the stack so that that the laser (24) heats the battery (18) and adjusts at least one parameter of the laser (24), characterized in that the parameter is set to a value which is the image by a mathematical model from minus a thermal characteristic of the cell (18).   Brazing method according to claim 1, in which the laser parameter is a parameter chosen from an irradiation time, an irradiation surface of the end face of the battery (18) by the laser (24). and an irradiation power of the laser (24).   Brazing method according to claim 1 or 2, wherein the thermal characteristic of the battery (18) is selected from a critical temperature of damaging an element (12, 14, 16) of the battery (18), a melting temperature of a solder mass (20, 22) interposed between two elements (12, -14, 16) of the cell (18) and a temperature of a synthetic material bordering one of the elements (12, 14); 16) of the battery (18).   4. A method of producing a mathematical model for implementing the soldering method according to any one of claims 1 to 3, characterized in that it comprises the following steps, according to an experimental plan: setting the value of the laser parameter (24), - measuring the temperatures over a zone (Z1, Z2, Z3) of at least one sample (28, 34, 36) of at least a portion of the stack ( 18) during a period of irradiation of a first end face (f = 1) of the sample (28, 34, 36) by the laser, selecting at least a remarkable temperature from the temperatures recorded and in that it also comprises a step of defining the mathematical model such that a vector having at least the coordinate of the value of the parameter of the laser is the image by the mathematical model of a vector having at least the coordinate of the remarkable temperature.   A method of making a mathematical model according to claim 4, wherein the mathematical model comprises at least one polynomial type mathematical function. -12-   6. A method of producing a mathematical model according to claim 4 or 5, wherein the zone (Z1, Z2, Z3) extends over a second end face (F2, F2 ', F2 ") of the sample (28, 34, 36) and the remarkable temperatures are maximum temperatures found in a remarkable subzone (SZ1A, SZ1B, SZ1C, SZ2A, SZ2B, SZ3) of this area.   The method of soldering according to claim 6, wherein the remarkable sub-area (SZ1A, SZ2A) separates a part (P1) from the zone (Z1, Z2) intended to be covered with a solder mass of another part (P2) of the zone (Z1, Z2) intended not to be covered with the solder mass.   8. A method of producing a mathematical model according to claim 6 or 7, wherein the sub-area is the projection (SZ1 B, SZ2B, SZ3) on the second end face (F2, F2 ', F2 "). of the sample (28, 34, 36) of the irradiation surface (S) of the laser.   9. A method of producing a mathematical model according to any one of claims 6 to 8, wherein, an element (12) of the sample (28) comprising a metal part (PM) bordered by a synthetic material part. (PS), the remarkable subfield (SZ1C) separates the metal part (PM) from the synthetic material part (PS).   10. A method of producing a mathematical model according to any one of claims 4 to 9, wherein, prior to the temperature reading step, the area (Z1, Z2, Z3) of the sample is blackened. (28, 34, 36).   11. A method of producing a mathematical model according to any one of claims 4 to 10, wherein the temperatures are read from a treatment of an infrared image of the zone (Z1, Z2, Z3) of the sample (28, 34, 36). 25   12. A method of producing a mathematical model according to any one of claims 4 to 11, wherein the experimental plane is a plane of the type selected from a Box Behnken plane, a composite central plane and a D-optimal plane.   13. A method of making a mathematical model according to any one of claims 4 to 12, wherein the mathematical model is determined from a method of analyzing the variance of the outstanding temperatures.   14. Electronic module (10) comprising a first support element (112) on which at least second (14) and third (16) electrical element elements are stacked, the three elements being brazed together, characterized in that the three elements elements (12, 14, 16) are brazed together by irradiation of the electronic module (10) with a laser.